This invention relates in general to power supplies and battery charging systems and, more particularly, to a battery charging system for portable electronic devices or electrically powered devices that can function as a universal power source for powering the portable device, charging the batteries, or both.
Portable electronic devices such as, for example, personal computers of the notebook, laptop, palmtop, or pen-based type, digital oscilloscopes, communications analyzers, surveying equipment, etc., usually have complex power supply requirements. For example, a portable computer typically drives a large display, a microprocessor, and a disk drive such as a floppy disk or hard drive, or both. Early "luggable" personal computers (PCs), used displays that were small cathode ray tubes (CRT) with floppy drives. Their power requirements were about 75 to 100 watts and required connection to a wall socket in order to meet such power requirements. Later, more portable computers, such as laptops, emerged. These newer systems eliminated CRTs and replaced them with low-power liquid crystal displays (LCDs); additionally, smaller disk drives requiring less space and power replaced the earlier larger drives. These newer systems reduced their power requirements sufficiently to enable them to operate on rechargeable batteries.
Generally, as shown in FIG. 1A, a portable computer 1 uses a power supply that includes an AC adapter 2 (an external AC/DC switcher), some form of limited current 3 to trickle charge the battery, steering diodes 4 to isolate the battery 5 from the raw supply voltage and an internal DC/DC converter 6. The DC/DC converter operates from either the power external supply or 6 to 10 internal nickel/cadmium (NiCad) batteries, which supply power for about one to two hours. This tends to restrict charging to trickle charge currents and wastes power in the steering diodes, reducing battery discharge time.
To charge the battery rapidly, careful attention must be paid to the level of charge to prevent overcharging the battery. A common method to detect full charge is known as "negative delta V" (-.DELTA.V). This method detects the small negative change in voltage across the NiCad cell as it becomes fully charged. To prevent voltage fluctuations across the battery that could lead to false detection, the rapid charge current must be held constant and cannot be a function of the computer load current; or, the battery must be disconnected from the charger while measuring -.DELTA.V as is done in the example shown in FIG. 1B.
A known power supply 10 is shown in FIG 1B. Power supply 10 includes an AC adaptor 12, which has a wide range AC input and is capable of providing 35 Watts (W) of DC output to the device and batteries 16. While charging, a current control maintains constant current to the combination of battery 16 and a DC-DC converter 14. In this manner, as more current is drawn by DC-DC converter 14, less current is supplied to charge the battery. Four wires, +VA, +VS, -VA and -VS connect AC adaptor 12 to the DC-DC converter 14 and batteries 16. The power converter topology includes a two-field-effect-transistor (FET) forward converter 18 with both a current control and a maximum voltage control. AC adaptor 12 uses a -.DELTA.V battery charging technique controlled by an internal processor 20. Power for DC-DC convertor 14 and battery charging is transferred over the +VA and -VA leads. Negative .DELTA.V it measured across leads +VS and -VS. To prevent false readings, a FET switch in the negative lead of the battery is turned off while -.DELTA.V is being measured. AC adaptor 12 reverts to a constant maximum voltage control when the battery becomes fully charged and is removed from the charging circuit by turning off the FET switch, or by physically removing the battery. To protect against over-charging the battery, AC adaptor 12 uses a battery-charge timer and a charge detector. The power-conversion circuits for the DC outputs are FET buck and boost converters operating at about 50 kilohertz (kHz). Because of the range of battery voltage while charging and discharging, DC-DC converter 14 is required to operate over a 2:1 voltage range. Typically, digital logic and analog application specific integrated circuits (ASICs) are used to minimize circuit space normally required for status and control signals.
This type of system, however, restricts rapid charging to periods when the computer is not being used and requires that the power supply/charging system be specifically designed for each battery/computer combination. Further, it is necessary to pulse the battery charging current off while -.DELTA.V measurements are being made.
Accordingly, what is needed is an improved power supply-battery charging system for portable electronic devices that is more efficient, thereby optimizing battery discharge time. In addition, what is needed is a system that reduces the need for steering diodes, thereby decreasing the minimum voltage usage necessary to power a device. This improvement should also improve rapid charging in either stand alone use, i.e., without the computer, or in the battery-computer combination during system operation, and should charge various battery types without modification to the charger.